1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and devices and, more particularly, to mechanisms and techniques for recharging a device that generates a subsea force.
2. Discussion of the Background
During the past years, with the increase in price of fossil fuels, the interest in developing new production fields has dramatically increased. However, the availability of land-based production fields is limited. Thus, the industry has now extended drilling to offshore locations, which appear to hold a vast amount of fossil fuel.
The existing technologies for extracting the fossil fuel from offshore fields may use a system 10 as shown in FIG. 1. More specifically, the system 10 may include a vessel 12 having a reel 14 that supplies power/communication cords 16 to a controller 18. A Mux Reel may be used to transmit power and communication. Some systems have hose reels to transmit fluid under pressure or hard pipe (rigid conduit) to transmit the fluid under pressure or both. Other systems may have a hose with communication or lines (pilot) to supply and operate functions subsea. However, a common feature of these systems is their limited operation depth. The controller 18 is disposed undersea, close to or on the seabed 20. In this respect, it is noted that the elements shown in FIG. 1 are not drawn to scale and no dimensions should be inferred from FIG. 1.
FIG. 1 also shows a wellhead 22 of the subsea well 23 and a drill line 24 that enters the subsea well 23. At the end of the drill line 24 there is a drill (not shown). Various mechanisms, also not shown, are employed to rotate the drill line 24, and implicitly the drill, to extend the subsea well.
However, during normal drilling operation, unexpected events may occur that could damage the well and/or the equipment used for drilling. One such event is the uncontrolled flow of gas, oil or other well fluids from an underground formation into the well. Such event is sometimes referred to as a “kick” or a “blowout” and may occur when formation pressure exceeds the pressure of the column of drilling fluid. This event is unforeseeable and if no measures are taken to prevent it, the well and/or the associated equipment may be damaged.
Thus, a pressure controlling device, for example, a blowout preventer (BOP), might be installed on top of the well to seal the well in case that the integrity of the well is affected. The BOP is conventionally implemented as a valve to prevent the release of pressure either in the annular space between the casing and the drill pipe or in the open hole (i.e., hole with no drill pipe) during drilling or completion operations. FIG. 1 shows BOPs 26 or 28 that are controlled by the controller 18, commonly known as a POD. The controller 18 controls an accumulator 30 to close or open BOPs 26 and 28. More specifically, the controller 18 controls a system of valves (not shown) for opening and closing the BOPs. Hydraulic fluid, which is used to open and close the valves, is commonly pressurized by equipment on the surface. The pressurized fluid is stored in accumulators on the surface and subsea to operate the BOPs. The fluid stored subsea in accumulators may also be used to shear and/or to support acoustic functions when the control of the well is lost. The accumulator 30 may include containers (canisters) that store the hydraulic fluid under pressure and provide the necessary pressure to open and close the BOPs. The pressure from the accumulator 30 is carried by pipe 32 to BOPs 26 and 28.
As understood by those of ordinary skill in the art, in deep-sea drilling, in order to overcome the high hydrostatic pressures generated by the seawater at the depth of operation of the BOPs, the accumulator 30 has to be initially charged to a pressure above the ambient subsea pressure. Typical accumulators are charged with nitrogen but as precharge pressures increase, the efficiency of nitrogen decreases which adds additional cost and weight because more accumulators are required subsea to perform the same operation on the surface. For example, a 60-liter (L) accumulator on the surface may have a useable volume of 24 L on the surface but at 3000 m of water depth the usable volume is less than 4 L. To provide that additional pressure deep undersea is expensive, the equipment for providing the high pressure is bulky, as the size of the canisters that are part of the accumulator 30 is large, and the range of operation of the BOPs is limited by the initial pressure difference between the charge pressure and the hydrostatic pressure at the depth of operation.
In this regard, FIG. 2 shows the accumulator 30 connected via valve 34 to a cylinder 36. The cylinder 36 may include a piston (not shown) that moves when a first pressure on one side of the piston is higher than a second pressure on the other side of the piston. The first pressure may be the hydrostatic pressure plus the pressure released by the accumulator 30 while the second pressure may be the hydrostatic pressure. Therefore, the use of pressured canisters to store high-pressure fluids to operate a BOP make the operation of the offshore rig expensive and require the manipulation of large parts.
As discussed above with regard to FIG. 2, the accumulator 30 is bulky because of the low efficiency of nitrogen at high pressures. As the offshore fields are located deeper and deeper (in the sense that the distance from the sea surface to the seabed is becoming larger and larger), the nitrogen based accumulators become less efficient given the fact that the difference between the initial charge pressure to the local hydrostatic pressure decreases for a given initial charge, thus, requiring the size of the accumulators to increase (it is necessary to use 16 320-L bottles or more depending on the required shear pressure and water depth), and increasing the price to deploy and maintain the accumulators.
As disclosed in U.S. patent application Ser. No. 12/338,652, filed on Dec. 18, 2008, entitled “Subsea Force Generating Device and Method” to R. Gustafson, the entire disclosure of which is incorporated herein, a novel arrangement, as shown in FIG. 3, may be used to generate the force F. FIG. 3 shows an enclosure 36 that includes a piston 38 capable of moving inside the enclosure 36. The piston 38 divides the enclosure 36 into a chamber 40, defined by the cylinder 36 and the piston 38. Chamber 40 is called the closing chamber. Enclosure 36 also includes an opening chamber 42 as shown in FIG. 3. The enclosure 36 may be formed in a BOP and the opening chamber 42 and the closing chamber 40 actuate the ram block (not shown) connected to rod 44.
The pressure in both chambers 40 and 42 may be the same, i.e., the sea pressure (ambient pressure). The ambient pressure in both chambers 40 and 42 may be achieved by allowing the sea water to freely enter these chambers via corresponding valves (not shown). Thus, as there is no pressure difference on either side of the piston 38, the piston 38 is at rest and no force F is generated.
When a force is necessary to be supplied for activating a piece of equipment, the rod 44 associated with the piston 38 has to be moved. This may be achieved by generating a pressure imbalance on two sides of the piston 38.
Although the arrangement shown in FIG. 3 and described in patent application Ser. No. 12/338,652, to R. Gustafson discloses how to generate the undersea force without the use of the accumulators, however, as discussed later, the accumulators still may be used to supply a supplemental pressure. FIG. 3 shows that the opening chamber 42 may be connected to a low pressure recipient 60. A valve 62 may be inserted between the opening chamber 42 and the low pressure recipient 60 to control the pressures between the opening chamber 42 and the low pressure recipient 60.
As shown in FIG. 3, when there is no need to supply the force, the pressure in both the closing and opening chambers is Pamb while the pressure inside the recipient 60 is approximately Pr=1 atm or lower to improve efficiency. When a force is required for actuation of a piece of equipment of the rig, for example, a ram block of the BOP, the seawater is prevented to enter the opening chamber 42 and valve 62 opens such that the opening chamber 42 may communicate with the low pressure recipient 60. The following pressure changes take place in the closing chamber 40, the opening chamber 42 and the low pressure recipient 60. The closing chamber 40 remains at the ambient pressure as more seawater enters via pipe 64 to the closing chamber 40 as the piston 38 starts moving from left to right in FIG. 4. The pressure in the opening chamber 42 decreases as the low pressure Pr becomes available via the valve 62, i.e., seawater from the opening chamber 42 moves to the low pressure recipient 60 to equalize the pressures between the opening chamber 42 and the low pressure recipient 60. Thus, a pressure imbalance occurs between the closing chamber 40 and the opening chamber 42 (which is now sealed from the ambient) and this pressure imbalance triggers the movement of the piston 38 to the right in FIG. 3, thus generating the force F.
One feature of the device shown in FIG. 3 is the fact that the low pressure recipient 60 has a limited functionality. More specifically, once the seawater from the opening chamber 42 was released into the low pressure recipient 60 and the opening chamber 42 was sealed from ambient, the low pressure recipient 60 cannot again supply the low pressure unless a mechanism is implemented to empty the low pressure recipient 60 of the received sea water. In other words, the seawater that occupies the low pressure recipient 60 after valve 62 has been opened, has to be removed and the gas at the atmospheric pressure that existed in the low pressure recipient 60 prior to opening the valve 62 has to be reestablished for recharging the low pressure recipient 60.
According to an exemplary embodiment and as shown in FIG. 4, the low pressure recipient 60 may be reused by providing a reset recipient 70 connected to the low pressure recipient 60, as described in U.S. patent application Ser. No. 12/338,669, filed on Dec. 18, 2008, entitled “Rechargeable Subsea Force Generating Device and Method” to R. Gustafson, the entire disclosure of which is incorporated herein. The reset recipient 70 and the low pressure recipient 60 may be formed integrally, i.e., in one piece. FIG. 4 shows the low pressure recipient 60 and the reset recipient 70 formed in a single reset module 72.
The low pressure recipient 60 may include a movable piston 74 that defines a low pressure gas chamber 76. This low pressure gas (or vacuum) chamber 76 is the chamber that is filled with gas (air for example) at atmospheric pressure and provides the low pressure to the opening chamber 42 of the BOP. The low pressure recipient 60 may include a port 78, which may be a hydraulic return port to the BOP.
A piston assembly 80 penetrates into the low pressure recipient 60. The piston assembly 80 is provided in the reset recipient 70. The piston assembly 80 includes a piston 82 and a first extension element 84. The piston 82 is configured to move inside the reset recipient 70 while the first extension element 84 is configured to enter the low pressure recipient 60 to apply a force to the piston 74. The piston 82 divides the reset recipient 70 into a reset opening retract chamber 86 and a reset closing extend chamber 88. The reset opening retract chamber 86 is configured to communicate via a port 90 with a pressure source (not shown). The reset closing extend chamber 88 is configured to communicate via a port 92 to the pressure source or another pressure source. The release of the pressure from the pressure source to the reset recipient 70 may be controlled by valves 94 and 96. A solid wall 98 may be formed between the low pressure recipient 60 and the reset recipient 70 to separate the two recipients. A second extension element 100 of the piston 82 may be used to lock the piston 82. The piston 82 may be locked in a desired position by a locking mechanism 102. Mechanisms for locking a piston are know in the art, for example, Hydril Multiple Position Locking (MPL) clutch, from Hydril Company LP, Houston, Tex. or other locking device such as a collet locking device or a ball grip locking device.
However, it would be desirable to provide other systems and methods for recharging the low pressure recipient.